1. Field of the Invention
The invention relates to coupling propulsion shafting to other drive train components and, more specifically, to couplings used in U.S. Navy propulsion systems with components made of composite materials.
2. Description of the Related Art
Propulsion shafts, often made of metal in the past, are today increasingly being made of composite materials. The composite materials may be laminated or filament-wound to form shafts. The shafts connect with motors, engines, generators, etc. to deliver torque to other drive-train components. Components are joined to drive shafts and other drive train components, such as flexible couplings, thrust bearings, gearboxes, prime movers, and propulsors to complete the drive train. Many of the components are often fabricated of metal. Thus, a shaft coupling is a necessary interface between two very distinct shaftline components.
Composite materials typically used in the fabrication of drive train components include polymer-based resin matrix materials, for example thermosetting epoxies and thermoplastic organic polymers, reinforced with a continuous organic fiber, such as continuous carbon fibers or continuous glass fibers. In an exemplary manufacturing process, a layered, filament-wound product is made by winding the organic fiber, saturated with the matrix material, onto a spinning mandrel to create the desired article. The organic fiber reinforces the layered article, with the reinforcement being contained within each layer.
There are only two approved methods for coupling the components of the drive train as currently practiced on U.S. military ships. The first is to provide shafting with flanges forged integrally. At the present time, however, it is not practical to filament wind a composite flange capable of handling ship propulsion loads. The second method is to utilize square keys and keyways to transfer the torsional load, a system which poses particular problems, especially with respect to propulsion systems combining composite and metal component materials.
The primary load for a Navy propulsion shaft is torsion. Composite structures, which are anisotropic materials, can be quite intolerant of high stress concentrations such as can occur in the vicinity of holes, fillets, notches, and grooves. U.S. Navy propulsion shafting must be capable of handling high torsional loads, extended bending-fatigue life in a seawater environment, and moderate axial loads to push or pull a vessel through the water.
Work in the application of composite propulsion shafting, conducted at the Naval Surface Warfare Center in Annapolis, has resulted in successful demonstrations of composite materials to carry the loads necessary. Filament wound composite tubes have been designed and fabricated to carry all of the above-mentioned combined propulsion loads. The most challenging work involves the design and demonstration of composite joining techniques capable of transferring the primary propulsion loads between the composite shafting and other shaft-line components.
Due to their anisotropic nature, fiber-reinforced composites can exhibit stress concentrations as high as 9. (Stress concentrations are unitless factors which are applied to "local" regions of a structure. The "local" regions exhibit an abrupt change in geometry, material or both. Instead of performing a detailed stress analysis for each "local" region, the "average" stress of the structure is multiplied by the stress concentration to give an approximate stress for the "local" region). Typical stress concentrations for isotropic materials range from 2 to 3. These stress concentrations result from holes, cut-outs or other dramatic changes in structural geometry.
Shafting generally exhibits a constant geometry along its length until a coupling is employed to couple powertrain components or additional shaft sections together. For example, current practice utilizes a steel shaft coupled to a bronze propulsor. Couplings are always accompanied by stress concentrations, whether it is due to a change in geometry or a change in materials. The problem is magnified with composite materials, due primarily to the extreme stress concentrations associated with these highly anisotropic materials.
Couplings have been standardized for conventional, forged steel, Navy propulsion shafting. Standard, metallic keyed couplings, use square keys and keyways mounted along the axis of the shaft. An example of a typical prior art coupling is shown in cross-section in FIG. 1. The keys transfer torsion through shear at the midplane of the key, or at the interface between the shaft and the coupling. The machined, square key systems induce stress concentrations where abrupt changes in geometry occur, typically at the corners of the key. This corresponds to the root or base of the keyway machined in the shaft.
A composite shaft comprises layered material. The layers are concentric rings of reinforced material, typically as shown in cross-section in FIG. 2. A composite shaft has no reinforcement, however, in the radial direction. For this reason, a typical mode of failure for a layered composite is delamination between the axially-reinforced layers. Square, machined keys and keyways in a layered, composite propulsion shaft generate peak stresses at the root of the keyway, then immediately drop to zero toward the bore of the shaft and rapidly drop to zero as one moves toward the midplane of the key. The high stress gradient promotes premature interlaminar failure.